A crystal having NLO (Nonlinear Optical) effect is called a NLO crystal. For the purpose of present invention, NLO effect means second harmonic generation (SHG) effect, sum-frequency generation (SFG) effect, difference-frequency generation (DFG) effect, parametric amplification (OPA) effect and the like. Only noncentrosymmetric crystal is possible to have NLO effect. NLO devices such as SHG device, up and down conversion device and optical parametric oscillation device, can be manufactured by utilizing the NLO effect of the crystal. A laser beam can be changed by frequency-conversion to result in more lasers within usable wavelength, thereby making a laser apparatus suitable for more usage. A blue-green laser system can be obtained by utilizing the conversion the frequency of the near-infrared laser produced by a solid-state laser apparatus by a NLO crystal, which has a numerous applications in the field of laser technology.
SHG is the most important NLO effect. An electromagnetic wave with a frequency of .omega. propagating in a NLO crystal will induce a polarization wave of a frequency of 2 .omega.. That is the so-called "SECOND HARMONIC GENERATION". The conversion efficiency of a SHG crystal is proportional to the effective SHG coefficient (d.sub.eff) square and the input laser power, and is also relative with the phase matching condition. When other conditions are selected, if phase matching is achieved, the conversion efficiency will reach the maximum. Phase matching occurs only for certain crystallographic directions, along which the refractive indices are the same for both the fundamental and the second harmonic waves. For a uniaxial crystal, phase matching relates exclusively to the angle .theta. which is the angle between the optical axis and the propagation direction of the incoming beam. For a biaxial crystal, phase matching relates to both .theta. and .PHI. which are the polar angles of the propagation direction of the incident wave. Phase matching can be achieved in various ways. We usually use the angle tuning method by rotation of the crystal to achieve it.
Currently, NLO materials used for blue-green laser frequency-conversion mainly include KTP (KTiOPO.sub.4)(U.S. Pat. No. 3,949,323), BBO (.beta.-BaB.sub.2 O.sub.4)(China Sciences, B28, 235, 1985), and LBO (LiB.sub.3 O.sub.5) crystal (Chinese patent application No. 88102084). These materials are defective on crystal growth because KTP and LBO are incongruently melting compounds and BBO undergoes phase changing below melting point. Accordingly, their growth depends on the use of flux method, which results in lower growth rate, higher cost and smaller size.
Development of new NLO materials suitable for being used in the blue-green laser apparatus faces a number of difficulties. Such crystals should be superior in both light transmission and workability. Preferably, the material is a congruently melting compound, so as to possibly grow a single crystal from melt and obtain NLO crystals with great size.